**5.3 Excavation works**

The depth of digging depends on the specifications of the soil. The inclination of the sides should be 30 cm for each meter depth for the cohesive soil, 60 cm to one meter for the light soil, and 90 cm for the sandy soil.

(a, b) BIOGAS PLANT DESIGN

Fig. 6. Excavation works for household units

The marking of dimensions (Fig. 5) for biodigester unit should be performed prior to start of excavation work. The marking is considered as preparation for excavation and construction works. An operational area of 3 m width around the digester should be considered, where the workers will use this area to achieve the construction works around the tank base in

The depth of digging depends on the specifications of the soil. The inclination of the sides should be 30 cm for each meter depth for the cohesive soil, 60 cm to one meter for the light

(a, b) BIOGAS PLANT DESIGN

order to prepare the structure of the concrete base, i.e. the bottom of the digester.

**5.2 Dimensions marking** 

Fig. 5. Marking of dimensions for a biodigester

Fig. 6. Excavation works for household units

**5.3 Excavation works** 

soil, and 90 cm for the sandy soil.

The bottom of the pit should be concave, where the center of the digester should be the most concave. Generally, the pit shape depends on the design of the digester, where in the case of round-shaped household units (Figs. 6a and b) a guide wood post is installed in the center of pit bottom. A string is linked to the post and used to set the round-shape of the pit. On the other hand, in the case of commercial biogas plants (Fig. 7), total station, teodolit or laser leveling is used for surveying. For large digesters, i.e. for commercial biogas plants, bulldozers are used to achieve the excavation (Fig. 7).

Fig. 7. Excavation works for a commercial biogas plant
